1. Field of the Invention
The present invention relates to an acoustic apparatus which is connected to a general-purpose or usual power amplifier for normally driving a loudspeaker and improve electro-acoustic reproduction (transduction) characteristics of an loudspeaker constituting the acoustic apparatus, and a driving apparatus for driving the loudspeaker to improve its electro-acoustic reproduction characteristics in cooperation with the general-purpose power amplifier.
2. Prior Art
A conventional power amplifier for driving a loudspeaker (loudspeaker unit (vibrator) or a loudspeaker system) normally has a substantially zero output impedance, and constant-voltage drives the loudspeaker.
In contrast to this, recently, there are proposed acoustic systems which improve acoustic reproduction characteristics of loudspeakers or make a loudspeaker vibration system compact without impairing acoustic reproduction characteristics by so-called negative impedance driving (a negative impedance component is included in an output impedance) or so-called motional feedback (MFB) driving (a loudspeaker output is detected by a certain method) (European Patent Application Publication No. 0 322 686, No. 0 322 679, No. 0 322 053, U.S. Pat. No. 4,118,600 and the like).
However, these acoustic systems need special-purpose driving apparatuses (power amplifiers) corresponding to loudspeakers to be used. For this reason, when a user who possesses and regularly uses a general-purpose power amplifier wants to constitute the system, he or she cannot utilize his power amplifier at all.
Japanese Patent Application Laid-Open Gazette No. Sho 58-29295 discloses a technique that a loudspeaker is connected in series with a negative resistance circuit having a negative resistance at a predetermined frequency or less, and the series circuit of the loudspeaker and the negative resistance is driven by a general-purpose amplifier to improve bass tone range characteristics of the loudspeaker. However, as shown in FIG. 9, the negative resistance circuit uniquely disclosed in the embodiment of Japanese Patent Application Laid-Open Gazette No. Sho 58-29295 is a transistor class-A single amplifier which has a resistor as a load. Such a transistor class-A single amplifier is not used for supplying power to a low-impedance load such as a loudspeaker in terms of voltage utilization efficiency, power loss, cost, and the like except for a case wherein it is used for a very small power such as an earphone or a headphone, or a very special case like in a hobby use. More specifically, this transistor class-A single amplifier does not belong to a category of power amplifiers for driving a loudspeaker in a general idea. Furthermore, in this negative resistance circuit, an impedance Z which is a detection resistor corresponding to a detection resistor Rs for speaker current detection according to the present invention, is connected in series with the loudspeaker unit through a transistor and, an emitter resistor is connected at one end to a point where the emitter of the transistor and loudspeaker unit 2 are connected each other and is also connected at its other end to a grounding point. With this arrangement, a current that flows through the loudspeaker (speaker current) also flows through the emitter resistor at the time when the loudspeaker is driven, whereby it is made impossible to precisely detect the current (speaker current) flowing through the loudspeaker unit. Since this negative resistance circuit is not constituted by a push-pull circuit and; among A class amplifiers, the negative resistance circuit is restricted in its applicability, it cannot simply be replaced with a power amplifier circuit which precisely detects the speaker current thereby to enable it to output the optimal negative resistance and is generally suitable for driving the speaker.
More specifically, although Japanese Patent Laid-Open Gazette No. Sho58-29295 theoretically suggests that a loudspeaker can be negative-resistance driven using a general-purpose power amplifier by connecting the loudspeaker in series with a negative resistance circuit, it does not disclose or suggest an arrangement of a practical negative resistance circuit which can drive a loudspeaker without posing any problem.